1. Field of the Invention
The present invention relates to an amplifier circuit with a shared current source, in particular to an amplifier circuit with multiple cascaded stages of amplifiers that share a common current source to reduce power dissipation.
2. Description of Related Arts
In general, amplifiers may have a gain stage and an output stage. However, the driving capability of the gain stage is usually not enough to meet high power requirements, and a Class A amplifier usually needs an output stage to boost the output power when the amplifier is linked to a large load. Class A amplifiers can be further divided into a source type or a sink type.
With reference to FIGS. 7 and 8, source type and sink type Class A amplifiers have a gain stage (51) and an output stage (52). The main distinction between these two types of amplifiers is that the source type can provide large current output to back-end circuits, whereas the sink type takes in current. The output stage in both types of amplifiers needs a current source (53) to drive the current output. When the source type amplifier is not active, the source circuit still draws substantial electrical energy from the current source (53). To improve the operation efficiency of the amplifier and to remove noise in the circuits, the current source (53) in a source type amplifier is usually designed with high current output, hence the power dissipation is even greater.
In certain drive circuits (such as LCD devices), incorporating both a source type and a sink type amplifier side by side is necessary. The common practice is to put these two types of amplifier circuits in independently. When these two amplifier circuits are not active, their total power dissipation is twice that of a shared current source (53) since each circuit has its own current source. Therefore the amplifier circuit design needs to be improved to meet the increasing demand to save power.